Some Vaccines Safe to Administer to Younger Foals

"Young horses have not had previous exposure to infectious agents so if they are challenged with a high level of pathogen and have not been vaccinated previously they could develop serious illness," Davis said.

Photo: Anne M. Eberhardt/The Horse

By the time a human infant reaches 4 months of age he or she has likely received a battery of vaccinations, starting shortly after birth. In contrast, four- to six-month-old foals whose dams were vaccinated properly are likely just starting to receive immunizations, as recommended by the American Association of Equine Practitioners' (AAEP) vaccination guidelines.

But what about foals born later in the year, in the midst of mosquito season? Should owners wait to vaccinate them until they reach four to six months of age? New research suggests that earlier vaccination—and, thus, earlier disease protection—might help ensure foals are well-protected against mosquito-borne disease.

At the 2013 AAEP Convention, held Dec. 7-11 in Nashville, Tenn., Elizabeth Davis, DVM, PhD, Dipl. ACVIM, presented research results indicating that veterinarians can begin providing foals protection against disease via vaccination sooner than once thought. Davis is a professor and section head of equine medicine and surgery at Kansas State University’s College of Veterinary Medicine.

In most cases, foals' born to well-vaccinated mares aren't vaccinated until maternal antibodies—which provide immune protection to the foal, but can also inhibit the foals’ ability to respond to vaccinations—contained in the mare's colostrum wane to a sufficient level), typically within six months after birth. Therefore, the AAEP currently recommends starting to vaccinate these foals for tetanus, Eastern and Western equine encephalomyelitis (EEE and WEE), West Nile virus (WNV), and equine herpesvirus (EHV) at 4 to 6 months of age, and rabies and influenza at 6 months of age.

To determine if foals might be able to receive protection against these potentially deadly diseases sooner than previously thought, Davis led a research team to compare foals vaccinated at 90 days of age versus those vaccinated at 180 days of age (6 months).

Davis and colleagues found that when foals received adequate colostrum and, therefore, had maternal antibodies present in circulation, those vaccinated with the Fluvac Innovator EHV1/4 vaccine and the West Nile-Innovator EWT vaccine (both produced by Zoetis) at three months of age showed similar immune response during the vaccination series and when boostered at 11 months when compared to foals initially vaccinated at 6 months of age.

She cautioned that "based on the data from this investigation and previous studies, tetanus and influenza vaccines are harder for foals to respond to in the face of maternal antibodies, and these two vaccines in particular should be administered at 4-6 months of age whenever possible to induce optimal immunity in the majority of foals. Those diseases that foals may get sick from during warmer months and are associated with mosquitoes such as EEE and WNV may be best initiated earlier in life (three to four months of age) so foals have their own immune protection initiated prior to viral exposure."

Davis said when using this earlier protocol, foals must receive a booster four weeks after the initial vaccine as well as a third vaccine 60 days later to complete the three-dose series. She said veterinarians should then administer the 11-month vaccine early in the foal's yearling year, followed by risk-specific vaccines, such as EEE, annually, adding that EHV-1/4 and influenza can be administered two to three times throughout the subsequent year.

She noted that the current AAEP vaccination guidelines should be followed for foals born early in the foaling season.

While concerns about overvaccinating horses have surfaced among owners recently, young horses should still receive vaccines to get a healthy start on life, Davis said.

"Infectious disease can be a serious problem—young horses have not had previous exposure to infectious agents so if they are challenged with a high level of pathogen and have not been vaccinated previously they could develop serious illness," she explained. "Two examples of such diseases include EEE and WNV, which are carried by mosquitoes and can infect naive horses. It is also important to note that even if foals have been vaccinated in the first year (foal/weanling age) it is important to continue with vaccines in the yearling year and thereafter to maintain effective immunity.

"Preventative health is important so horses can thrive and have a good quality of life," Davis concluded. "They live in an environment that contains many challenges and through effective vaccine protocols we can minimize the risk that they will suffer from disease."

About the Author

Erica Larson, news editor, holds a degree in journalism with an external specialty in equine science from Michigan State University in East Lansing. A Massachusetts native, she grew up in the saddle and has dabbled in a variety of disciplines including foxhunting, saddle seat, and mounted games. Currently, Erica competes in eventing with her OTTB, Dorado.

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